ajax error 500

Hi,

I really like this plugin and does exactly what it need to do for our contes we want to run. But after installing the plugin and making a page with the shortcode the submit button won’t work. So i looked in the console what happened and i get an Ajax 500 error after clicking the submit button. The error can be seen here: https://ibb.co/V3DRqvQ

I Use the latest wordpress version and PHP 7.4 on my server. How can i fix this?

Kind Regards